Concrete durability in northern climates is greatly dependant on the concrete's ability to maintain its integrity under freeze/thaw cycles. The engineering and concrete industries have established a relationship between concrete durability during temperature cycles and the air-void content of the concrete. Aggregate concentration and distribution is also a factor.
Air-voids are small hollow spaces enclosed in the cement paste of the concrete. Air-voids are generally larger than two micrometers in diameter. The term air-void includes both "entrapped" and "entrained". Entrained air-voids are spherical in shape, while entrapped air-voids are irregularly shaped and often result from improper consolidation. Aggregates are solid material in the concrete, such as stones.
The inspection of air-void content in concrete is usually performed in accordance with the American Society of Testing and Materials (ASTM) standard ASTM C 457-82a, which is entitled, "Standard Practice for Microscopical Determination of Air-Void Content and Parameters of the Air-Void System in Hardened Concrete," which publication is well-known in the art of concrete inspection and to which reference may be made for further details on the methodology and calculation of air-void content analysis with regard to the system of the present invention. The ASTM has established two primary procedures for determining air-void content in hardened concrete. The linear traverse method entails examining microscopically a finely ground section of concrete on a series of regularly spaced lines of traverse. The operator records the beginning and ending coordinates of air-voids and/or aggregates encountered along the traverse lines. This data yields information on the total number of sections of air-voids intersected by the microscope cross hairs, the total distance traversed across sections to voids, the total distance traversed across the remainder of the concrete, and the total distance traversed across the paste and aggregates.
The point count and modified point count methods entail microscopic examination of the concrete section along a similar series of regularly spaced traverse lines. The specimen is moved under the microscope at evenly spaced intervals along the traverse lines. The operator records the frequency at which air-voids or aggregates are encountered.
In the past, examining concrete specimens for their compliance with ASTM C 457 requirements was time consuming, inaccurate and expensive. Typically, the specimen would be attached to a positioning table under a microscope. The operator would record the location of the specimen in relation to the microscope cross hairs at the beginning of a traverse line. The operator would next manually reposition the table until an air-void or aggregate was encountered. Another data point would then be recorded. Again, the operator would reposition the table to the other edge of the air-void and record the position. Repeating this process for over 1,000 data points was extremely tedious and time consuming.
A number of devices are available for accelerating this process. The Trilogy Systems Model TS 600 Concrete Inspection System utilizes a joy stick to position the table containing the specimen. Upon reaching the start of an air-void, the operator presses a computer key for the data point type (i.e., air-voids or aggregates) and to prompt a computer to record the location. The operator then moves the table using the joy stick to the end of the air-void and again presses a button which instructs the computer to record the ending data point for the air-void.
The model MCS-83 computer controlled inspection system from Frank E. Fryer Co. utilizes a computer controlled stage which determines traverse line spacing, traverse length, and the number of points counted, as well as the distance between the points. The operator activates a counter mechanism at the beginning and ending of air-voids. Scanning speed is operator controlled in either manual positioning or automatic mode.